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January 15, 2021
by Lawrence Berkeley National Laboratory
A new study by a theoretical physicist at the US Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) suggests that never-before-observed particles called axions are the source of inexplicable high-energy X-ray emissions in the vicinity of Can be a group of neutron stars.
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First theorized in the 1970s as part of a solution to a fundamental particle physics problem, it is expected that axions are generated in the core of stars and, in the presence of a magnetic field, can transform into light particles called photons.
Axions can also make up dark matter – the mysterious material that is estimated to make up 85 percent of the total mass of the universe, but we have only seen its gravitational effects on ordinary matter so far. Even if the X-ray excess turns out not to be axions or dark matter, it could nonetheless reveal new physics.
A collection of neutron stars known as the Magnificent 7 provided an excellent test bed for the possible presence of axions as these stars Have strong magnetic fields, be relatively close – within hundreds of light years – and expect little production – energy x-rays and ultraviolet light.
« They are known to be very ‘boring' », and in this case it is a good thing, said Benjamin Safdi, a division member of the Berkeley Lab Physics Division’s theory group who led a study published Jan. 12 in the journal Physical Review Letters Listing the Axion Explanation for the Excess / p> Christopher Dessert, a subsidiary of the Berkeley Lab Physics Division, made an important contribution to the study, which Fo rers from UC Berkeley, the University of Michigan, Princeton University, and the University of Minnesota attended.
If the neutron stars were of a type known as pulsars, they would have an active surface that emits radiation at different wavelengths . This radiation would show up in the entire electromagnetic spectrum, said Safdi, and could drown out the X-ray signature found by the researchers or generate high-frequency signals. However, the Magnificent 7 are not pulsars and no such radio signal has been detected. Other common astrophysical explanations don’t seem to stand up to the observations either, said Safdi.
If the excess x-ray seen around the Magnificent 7 is generated by an object or objects hiding behind the neutron stars, it would likely have appeared in the datasets, The researchers use two space satellites: the European Space Agency’s XMM-Newton and NASA’s Chandra X-ray telescopes.
Safdi and co-workers say it is still entirely possible that a new explanation without an axion could arise about the X-ray excess observed although they remain confident that such an explanation will be outside the Standard Model of Particle Physics and this new foundation – and space-based experiments will confirm the origin of the high-energy X-ray signal.
« We are pretty confident that this excess exists , and very z confident that there is something new in this surplus, « said Safdi. « If we were 100% sure that what we see is a new particle, it would be huge. That would be revolutionary in physics. » Even if the discovery turned out not to be related to a new particle or a new dark matter, he said, « It would tell us so much more about our universe and there would be a lot to learn. »
Raymond Co, a University of Minnesota postdoctoral fellow who worked on the study, said, « We’re not saying we have already made the discovery of the axion, but we’re saying the extra X-ray photons can be explained. It’s an exciting discovery of the excess in the X-ray photons, and it’s an exciting possibility that already coincides with our interpretation of axions. «
If axions exist, they are expected to behave similarly to neutrinos in a star, since both have very low masses and interact only very rarely and weakly with other matter. They could be produced in abundance inside stars. Uncharged particles, called neutrons, move within neutron stars and occasionally interact by scattering each other and releasing a neutrino or possibly an axion. The neutrino-emitting process is the dominant way that neutron stars cool down over time.
Like neutrinos, the axions could move outside the star. The incredibly strong magnetic field that surrounds the Magnificent 7-Stars – billions of times stronger than magnetic fields that can be created on Earth – could cause exiting axions to be converted into light.
Neutron stars are incredibly exotic objects, and Safdi noted that a lot of modeling, data analysis, and theoretical work went into the latest study. Researchers have made heavy use of a bank of supercomputers known as the Lawrencium Cluster in the Berkeley Lab in their latest work.
Some of this work was done at the University of Michigan, where Safdi had previously worked. « Without the high-performance supercomputing work in Michigan and Berkeley, none of this would have been possible, » he said.
« A lot of data has been processed and data analyzed. You have to model the inside of a neutron star to predict how much Axions should be created in this star. «
Safdi noted that white dwarf stars would be a prime location for finding axions as the next step in this research, as they also have very strong magnetic fields and are likely to be » X-ray free environments » .
« It’s pretty convincing that this is a bit beyond the Standard Model, if we see excess X-ray there, too, » he said.
The researchers could use another X-ray space telescope called NuStar to solve the puzzle to solve the X-ray excess.
Safdi said he is also looking forward to ground-based experiments such as CAST at CERN, which serves as a solar telescope to detect axions caused by a powerful Magn eten would be converted into X-rays, and ALPS II in Germany, which would use a strong magnetic field, would cause axions on one side of a barrier to be converted into light particles when laser light hits the other side of the barrier.
Axions have more attention obtained as a series of experiments found no evidence of the WIMP (weakly interacting massive particles), another promising candidate for dark matter. And the axion picture isn’t that simple – it could actually be a family album.
There could be hundreds of axion-like particles or ALPs that make up dark matter, and string theory – a candidate theory used to describe the forces of the universe – keeps open the possible existence of many types of ALPs.
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